Atomic clocks have been conventionally used for a reference time standard. The atomic clocks have been generally accepted as representative ones of high precision clocks and have been adopted for primary references of time as an example. On top of that, the atomic clocks have been applied to fields that require high preciseness such as GPS (Global Positioning System), where each GPS satellite is equipped with an atomic clock. Generally speaking, atomic clocks utilize electromagnetic waves in a microwave region for a frequency reference, which waves are produced through transitions between electronic levels (hereinafter called “clock transitions”) in atoms (hereafter including ions) such as Cs (caesium) and Rb (rubidium).
Moreover, miniaturization of atomic clocks are also envisioned because atomic clocks with portability would be applied broadly. For example, an atomic clock called CSAC (Chip Scale Atomic Clock) is currently commercially available. CSACs with volume of ˜16 cm3 with frequency uncertainty of ˜10−11 have been developed.
Furthermore in another R&D trend, optical atomic clocks have been developed for the purpose of time measurement and frequency reference with higher precision than atomic clocks, where laser lights with high stable frequency are used for optical transitions of isolated atoms or ions in free space. However, it is not easy to obtain portability in such optical atomic clocks because they are equipped with ultra-stable laser light sources that use bulky vacuum facility equipment and anti-vibration devices of high precision.
Likewise, a clock called optical lattice clock has been developed for the superb preciseness over conventional atomic clocks and have gathered much attention for their operational principle (see for example Non-Patent Document 1 for a review article). In general, the optical lattice clocks use a spatial periodic structure through a standing wave of light or electromagnetic wave (hereinafter called “optical lattice”) and utilize clock transitions of atoms trapped at antinodes (lattice points) of the standing wave of the optical fields in the optical lattice. It is to be noted that the optical lattice clocks may be referred to as examples of atomic clocks or optical atomic clocks. In the present patent application, however, an optical lattice clock denotes an atomic clock that utilizes the optical lattice, and thus the optical lattice clocks are distinguished from atomic clocks or optical atomic clocks.